A liquid crystal display device is preferably driven by AC voltage to lengthen a useful life of liquid crystal material.
However, since a parasite capacitance Cgd exists between a gate electrode and a drain electrode as shown in FIG. 9(a), a voltage level Vs written into a source electrode is drawn and varies (ΔV) when the gate electrode switches from ON (Vgon) to OFF (Vgoff), resulting in decrease to Vd, as shown in FIG. 9(b). This variation is expressed by the following equation:ΔV=Cgd/(Clc+Ccs+Cgd)×(Vgon-Vgoff),
where Clc is liquid crystal capacitance, and Ccs is storage capacitance.
Therefore, in view of ΔV, a common electrode voltage (Vcom) and a source electrode voltage (Vs) must be adjusted by shifting their respective center values.
To a liquid crystal layer of each pixel, an electric potential difference between the common electrode voltage (Vcom) and the source electrode voltage (Vs) is supplied as a liquid crystal drive voltage. For this reason, when a variation ΔV occurs between a center of a voltage waveform for the common electrode and a center of a voltage waveform for the source electrode constituting a pixel electrode, difference in amplitude between the positive side and the negative side in a voltage waveform causes a variation of the liquid crystal drive voltage, resulting in the occurrence of flickers.
Further, a value of the liquid crystal capacitance Clc varies in black display and in white display, which causes variations ΔV of different values. This requires further adjustment.
In this connection, in the halftone display shown in FIG. 10, a voltage waveform of a voltage applied to the common electrode or a voltage waveform of a voltage applied to the source electrode must be shifted to match the center (Vcom1) of a voltage waveform of a voltage applied to the common electrode (Vcom) with the center (Vs1 or Vs2) of a voltage waveform of a voltage applied to the source electrode (Vs) for preventing the occurrence of flickers caused by variations of the liquid crystal drive voltage.
A conventional liquid crystal display device 50, as shown in FIG. 11, includes an offset circuit 52, connected to a common electrode drive circuit 53, which is provided with a variable resistor 51. For example, Japanese Laid-Open Patent Application No. 295164/1994 (Tokukaihei 6-295164; published on Oct. 21, 1994) discloses a liquid crystal display device which can adjust a voltage applied to a common electrode by the use of a variable resistor.
The value of the foregoing ΔV varies due to the variations of panels caused in the manufacturing process and other reasons. Therefore, an optimum applied voltage is applied to the common electrode in such a manner that the variable resistor 51 in the offset circuit 52 is independently adjusted to match the center of the voltage waveform for the common electrode with the center of the voltage waveform for the source electrode. This enables a shift in waveform of the common electrode voltage, thus preventing the occurrence of flickers.
However, the foregoing conventional liquid crystal display device 50 has the following problems.
That is, for reduction of power consumption, has been proposed in recent years a semi-transmissive liquid crystal display device including a plurality of display modes such as reflective mode in which external light is utilized for displays with a back light (hereinafter referred to as BL) turned off and a transmissive mode in which BL is utilized for displays.
As to a liquid crystal display device for displaying images in one display mode such as transmissive mode, no problems occur when an optimum applied voltage is once set because change of a display mode is not required. However, as to a liquid crystal display device capable of displays in a plurality of modes, change of a display mode causes change in liquid crystal capacitance Clc due to a different light propagation route. This causes increase in the value of ΔV, as compared to the value of ΔV obtained by the foregoing equation. Accordingly, the amount of drawn voltage in the source electrode voltage increases, resulting in a shift of the center of the voltage waveform for the source electrode voltage.
For example, as shown in FIG. 10, when the center of the voltage waveform for the common electrode voltage (Vcom) before shifted is Vcom1, and a center of an optimum voltage of the source electrode voltage (Vs) is Vs1 in the transmissive mode, the center Vs1 of the optimum voltage of the source electrode voltage shifts to Vs2 when the display mode is switched to the reflective mode. On this account, a value of the optimum voltage also shifts under the condition where setting is carried out in accordance with a display mode before switched, resulting in the occurrence of flickers.
Such a variation about the center of the voltage waveform, i.e. a variation about an optimum applied voltage, caused by switch of the display mode is as much as 0.1V to 0.2V, which is considerable. Therefore, in order to prevent the occurrence of flickers caused by switch of the display mode, the optimum applied voltage applied to the source electrode must be set again after the display mode is switched to match Vcom1 with Vs2.
However, in the conventional liquid crystal display device 50, once the optimum applied voltage is set by adjusting a resistance value of the variable resistor 51 in the offset circuit 52, which is mounted to generate the optimum applied voltage for the prevention of flickers, it is impossible to change the optimum applied voltage by readjusting the resistance value of the variable resistor 51 during operation of the liquid crystal display device 50. That is, it is impossible to prevent the occurrence of flickers caused by switch of the display mode carried out during operation of the liquid crystal display device 50.